[Design] - Prototype of ResourceModel

/cc @yishaigalatzer @harshgMSFT 

Not included here:
- Model Metadata integration
- Sample generation interface
- Filter interface for customizing formatters

Where possible I made decisions that are similar to webapi api explorer. See comments in the code for context on specific decision points and open questions.
This has no parallel in old-webapi, but it's extremely useful for grouping and filtering.